Current Drug Metabolism - Volume 2, Issue 1, 2001

The activation of molecular oxygen by Cytochromes P450 to the ultimate mono-oxygen oxidant species involves three distinct dioxygen species coordinated to the heme iron. These intermediates have different chemical properties, and have recently been proposed to participate in some Cytochrome P450- catalyzed oxidation reactions. This article reviews the extent of our current knowledge on the roles proposed for the heme- peroxo, hydroperoxo, and superoxo complexes in various reactions. The extent to which such species contribute to the breadth of reactions catalyzed by Cytochrome P450 has yet to be defined, and more definitive experiments are needed to establish such species in the reactions they are proposed to effect.

The most common drug-drug interactions may be understood in terms of alterations of metabolism, associated primarily with changes in the activity of cytochrome P450 (CYP) enzymes. Kinetic parameters such as K m , V max , K i and K a , which describe metabolism-based drug interactions, are usually determined by appropriate kinetic models and may be used to predict the pharmacokinetic consequences of exposure to one or multiple drugs. According to classic Michaelis-Menten (M-M) kinetics, one binding site models can be employed to simply interpret inhibition (pure competitive, non-competitive and uncompetitive) or activation of the enzyme. However, some cytochromes P450, in particular CYP3A4, exhibit unusual kinetic characteristics. In this instance, the changes in apparent kinetic constants in the presence of inhibitor or activator or second substrate do not obey the rules of M-M kinetics, and the resulting kinetics are not straightforward and hamper mechanistic interpretation of the interaction in question. These unusual kinetics include substrate activation (autoactivation), substrate inhibition, partial inhibition, activation, differential kinetics and others. To address this problem, several kinetic models can be proposed, based upon the assumption that multiple substrate binding sites exist at the active site of a particular P450, and the resulting kinetic constants are, therefore, solved to adequately describe the observed interaction between multiple drugs. The following is an overview of some cytochrome P450-mediated classic and atypical enzyme kinetics, and the associated kinetic models. Applications of these kinetic models can provide some new insights into the mechanism of P450-mediated drug-drug interactions.

Molecules exist as three dimensional structures. Therefore they can exist in symmetrical and asymmetrical forms. Molecules with an asymmetric centre are chiral. If the molecule and its mirror image are non-superimposable, the relationship between the two molecules is enantiomeric and the two stereoisomers are enantiomers. Since enantiomers have very similar or identical physicochemical properties, it is very difficult to distinguish between them in an achiral environment. However, once in a chiral environment, as in the body, they exhibit clear differences. In fact, most of the physiological processes in nature are stereospecific. Stereospecificity can occur in pharmacokinetic processes, in particular that utilise a carrier protein, receptor or enzyme. In addition, stereoselectivity occurs in pharmacodynamic processes and the differences between enantiomers can be either qualitative and quantitative. 2-arylpropionic acid derivatives (2APAs - profens) are an important subgroup within the class of NSAIDs . These are chiral compounds marketed as racemic mixtures. Some members of the group in an species-dependent manner undergo a special type of metabolic transformation leading to partial inversion to the optical antipode through a specific conjugation with CoA (coenzyme A) and subsequent epimerization. This metabolic inversion has not only pharmacological consequences (related to clinical effect) but also toxicological consequences such as, formation of hybrid triglycerides and even inhibition of fatty acid β-oxidation. Differences on inversion rate between compounds and species will be discussed as well as its modification by different patho-physiologic processes such as, inflammation.

There is usually considerable variability in anticancer drug plasma levels when delivered at high doses requiring stem-cell support. Given their narrow therapeutic windows and wide interpatient pharmacokinetic variability, drug monitoring and pharmacokinetic-directed dosing represent an attractive strategy in this setting. A major previous requirement to successful application of therapeutic drug monitoring is identification of a significant and clinically meaningful pharmacodynamic correlation between a pharmacokinetic parameter and a toxic or therapeutic outcome, or preferably, both. In this review, we will analyze the current knowledge of identified pharmacodynamic correlations in high-dose chemotherapy. We will summarize the observations from other authors and our own, on drugs employed at high doses, such as cyclophosphamide, melphalan, busulfan, carmustine, paclitaxel, or docetaxel.

Bestatin, an antibiotic of microbial origin, is a potent inhibitor of some, but not all aminopeptidases. It can be administered, with low toxicity, to cultured cells, intact animals and humans. It has become a useful tool in elucidating the physiological role of some mammalian exopeptidases in the regulation of the immune system, in the growth of tumors and their invasion of surrounding tissues, and in the degradation of cellular proteins. Bestatin-sensitive enzymes play important roles in the digestion and absorption of peptides in the brush border of the intestine and the kidney, in the reproductive system, and in the metabolism of opioid peptides and leukotrienes. Aminopeptidase N emerges as the major target for the effects of bestatin on the immune system and some of its effects on tumor growth and the endometrium. It is also the major bestatin-sensitive enzyme involved in the degradation of oligopeptides on the surface of intestine and kidney brush borders, and the inactivation of enkephalins in the brain. Bestatin-sensitive cytosolic exopeptidases are important in the degradation to amino acids of di- and tripeptides generated in most cells by cellular protein degradation, as well as those absorbed through the brush border of intestine and kidney. Inhibition of one of these exopeptidases, cytosol alanine aminopeptidase, results in apoptosis. Bestatin-sensitive cystinyl aminopeptidase is abundant in placenta. Two bestatin-sensitive enzymes, aminopeptidase B and nardilysin, are particularly abundant in late spermatids. Finally bestatin-sensitive LTA4 hydrolase generates the potent chemotactic agent, LTB4.